1. Field of the Invention
The present invention relates to a cylinder direct injection type internal combustion engine using two kinds of fuels that are different in ignitability.
2. Description of Related Art
A related art internal combustion engine uses low-octane fuel (high-cetane fuel) having a high self-ignitability and high-octane fuel having a high resistance to knocking. During a low-load operation, stable self-ignition combustion (lean combustion) is achieved by increasing the supply ratio of low-octane fuel. In contrast, during a high-load operation, pre-ignition is prevented by increasing the supply ratio of high-octane fuel. When two kinds of fuels are injected, a region where the fuel concentration is high (rich region) is formed, and an increase in combustion temperature in the region increases the amount of NOx emission. Moreover, the amount of smoke emission is increased by a shortage of air in the region. In order to solve these problems, low-octane fuel is injected toward the center of a combustion chamber and high-octane fuel is injected toward the periphery of the combustion chamber so that the low-octane fuel and the high-octane fuel are not mixed in the combustion chamber.
However, even when two kinds of fuels that are different in ignitability are separately injected toward the center and the periphery of the combustion chamber, as in the related art, the fuels need to be close to each other so that combustion is continuously performed, and it is difficult to completely separate the fuels. Since a rich region is inevitably formed, it is difficult to reduce the amount of NOx emission.